Program Problem

How do I go about setting up the rental upgrade part of program. Here is what I need to do:

The program will try to sell the next more expensive car if S or O is entered, by quoting the charge for the next more car less 5% discount on the entire price. For example, if the user asks foe an 'O' quote, the upgrade is to 'S' or upgrade to 'P' if asked for 'S'. Here is what I have so far.